Benzyl­ammonium hepta­noate

aBP Institute and Department of Chemistry, University of Cambridge, Cambridge, England*Correspondence e-mail: stuart@bpi.cam.ac.uk

(Received 20 February 2013;accepted 8 April 2013;online 20 April 2013)

The title 1:1 stoichiometric salt, C7H10N+·C7H13O2−, is formed by proton transfer between hepta­noic acid and benzyl­amine. This combination contrasts to the recently published 2:1 acid–amine adduct of cation, anion and neutral acid molecule from the same components [Wood & Clarke (2013). Acta Cryst. E69, o346–o347]. There are N—H⋯O hydrogen bonds of moderate strength in the structure [the most important graph-set motifs are R24(8) and R44(12)], as well as weak C—H⋯O inter­actions.

There exist in the literature a number of examples of complexes formed between alkyl amines and carboxylic acids. These have been identified by a number of experimental methods, such as NMR and IR spectroscopy (e. g. Karlsson et al., 2000; Paivarinta et al. (2000); Smith et al. (2001, 2002)). Unfortunately, the atomic details of the materials and hence the nature of the bonding that can be obtained from single-crystal diffraction has only been determined in a few cases. This is partly due to the challenges in growing crystals large enough to be suitable for single-crystal diffraction. Some crystals of sufficient size have been grown (e. g. Jefferson et al. (2011); Wood & Clarke, 2012a, 2012b). Of the stoichiometic combinations reported to date, the majority have been 1:1 complexes, though some 2:1 and even 3:1 examples have been reported by calorimetry and NMR spectroscopy, generally when the environment is acid-rich (e. g. Kohler et al., 1981; Sun et al., 2011), and very recently using single-crystal diffraction (Wood & Clarke, 2013).

Here we report growth of a suitable crystal for single-crystal X-ray diffraction of a 1:1 complex formed between heptanoic acid and benzylamine (Figs. 1 and 2). This work follows a previous publication (Wood & Clarke, 2013) in which an acid-rich 2:1 complex formed between these two species is described. In the previous work, one acid molecule donates its proton to the amine group and one acid group retains its proton. The hydrogen bonding extends across both the ions and the neutral acid molecule.

As described below, the sample of the present study was grown by vapour phase condensation. Each preparation resulted in a batch of several crystals. The crystal used in this present study was taken from a different region of the same batch of samples as for the 2:1 combination (Wood & Clarke, 2013). We attribute the combination of compositions to the concentration gradients across the vapour streams in the preparation.

In the crystal structure of the 1:1 complex (Figs. 1 and 2), each acid anion is involved in N-H···O hydrogen bonds of moderate strength (Gilli & Gilli, 2009) with three surrounding amine molecules (Table 1; Fig. 2), and vice versa each benzylammonium group donates its hydrogen to three different heptanoate molecules. The most important graph set motifs are R24(8) (Etter et al., 1990) - see Fig.3. (In this motif the donated atoms are H1a and H1c and the acceptors are the O2 atoms.) The other important motif is R44(12) with donated hydrogens H1b and H1c and accepting oxygens O1 and O2 (Fig. 3.). Moreover, there are also weak C-H···O interactions present in the structure (Table 1).

Benzylamine and heptanoic acid (purities 99.7% and 99.8% respectively, determined by titration and gas chromatography) were purchased from Sigma Aldrich and used without further purification. A small volume of amine (approximately 1 ml) was placed into a small vial that was itself placed within a larger vial containing a similar volume of the acid, and left in an inert atmosphere. Extensive crystal growth was observed after a few weeks, particularly on a polypropylene surface included in the vial to encourage nucleation.

All the hydrogens were discernible in the difference electron density map. Nevertheless, the hydrogens attached to the C atoms were situated in the idealized positions and refined under these constraints: Caryl-Haryl=0.95, Cmethyl-Hmethyl=0.98, Cmethylene-Hmethylene=0.99 Å. Ueq(Haryl)=1.2Uiso(Caryl), Ueq(Hmethylene)=1.2Uiso(Cmethylene), Ueq(Hmethyl)=1.5Uiso(Cmethyl). The positional parameters of the hydrogens attached to N of the benzylammonium cation were freely refined while their Ueq(HN)=1.5Uiso(HN).

Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes.

Refinement. Refinement of F2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F2, conventional R-factors R are based on F, with F set to zero for negative F2. The threshold expression of F2 > σ(F2) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F2 are statistically about twice as large as those based on F, and R- factors based on ALL data will be even larger.

Acknowledgements

We thank the Department of Chemistry, the BP Institute and the Oppenheimer Trust for financial and technical assistance, and Dr J. E. Davies and Dr A. D. Bond for assistance in collecting and analysing the X-ray data.

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